Rolling mill gauge control method and apparatus including entry gauge correction

ABSTRACT

An automatic gauge control is disclosed to provide on line control of the delivery gauge or thickness from at least one roll stand of a tandem rolling mill, for which one roll stand it is not desired to adjust the roll opening for the purpose of controlling the delivery gauge of workpiece strip leaving that roll stand. The gauge error of the workpiece strip leaving that one roll stand is corrected by predetermined adjustment of a selected previous roll stand in relation to the work-piece gauge error measured at that one roll stand and to provide a desired entry gauge correction in relation to that one roll stand.

United States Patent 1 [III 3,841,123

Fox et al. 1 Oct. 15, 1974 I54] ROLLING MlLl, GAUGE CONTROL 3.406.547 10/1968 List 72/X TH AND APPARATUS INCLUDING 3.561.237 2/1971 Eggcrs c1 :Il. 3.709.008 l/I973 SmIth. .Ir 72/8 ENTRY GAUGE CORRECTION Primary Examiner-Milton S Mehr Attorney, Agent, or Firm-R. G. Brodahl [57] ABSTRACT 7 An automatic gaugecontrol is disclosed to provide on line control of the delivery gauge or thickness from at least one roll stand of a tandem rolling mill, for which one roll stand it is not desired to adjust the roll opening for the purpose of controlling the delivery gauge of workpiece strip leaving that roll stand. The. gauge error of the workpiece strip leaving that one roll stand is corrected by predetermined adjustment of a selected previous roll stand in relation to the work-piece gauge error measured at that one roll stand and to provide a desired entry gauge correction in relation to that one roll stand.

9 Claims, 9 Drawing Figures [75] Inventors: Richard Q. Fox; Andrew W. Smith,

I I Jr., Pittsburgh, both of Pa.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: Nov. 6, 1972 [21] Appl. No.: 303,721

[52] US. Cl; 72/8, 72/16 [51] Int. Cl B2lb 37/02 [58] Field of Search 72/6-12, 16

[56] References Cited UNITED STATES PATENTS 2,680,978 6/1954 Hessenberg et al 72/9 3,232,084 2/1966 Sims 72/16 3,357,217 12/1967 Wallace et al. 72/8 SCREWDOWN POSITIONING 55 CONTROL .495 292i I 33 3| SCREWDOWN POSITION DETECTOR EWDOWN ITIONING ONTR'JL SCR POS C MILL ENTRY TEMPERATURE DETECTOR I 4] INFORMATION INPUT DEVICES OPERATOR STATION CONTROL PANEL DISPLAY UNITS, PRINTOUT DEVICES. I ETC. 1

PAIENIEII BUT I 5W4 3.841 1 23 SIIEU 2 0F 4 w I O m I E I FIG. 2 l I O I I I UNLOADED ROLL DELIVERY ENTRY OPENING GAUGE GAUGE (SDREF) 0) ("5) I Asn 111 I04 I02 g I E FIG. 3 II 0 0:

GAUGE /ERROR scar-:woowm 1 REFERENCEISDREFIJ PRESENT GAUGE(HX) PRESENT SCREWDOWN($D)* DESIRED GAUGE (H PAIENTEBBBT I 51914 3.941.123

LLI (J FIGS U.

. l l O [I Jr. I. ASDN-I H (N-l) HD(NI )-ENTRY GEIM- D PRESENT FORCE(F) LOCK-ON FORGEILOF) I FIG.6

LOCK-ON scREwDOwNILOsD)' PREsENT GAUGE( Hx) PRESENT SCREWDOWMSD) LOCK-ON GAUGE(HD) ROLL ROLL STAND(NI) STAND(N) 50 F K P sD F K P IN-I) IN-IIIN-I) IN-I) (N) (N) (N) (N) k I k ExIT GAUGE EXIT GAUGE ERROR DETERMINED ERROR DETERMINED AT STANDIN-I) AT sTAND (N I an GEN-U1 EXITGEIN) STAND(N-I)CORRECTION STAND NIGORREOTION FOR sTAND IN-IIExIT FORs AND(N)EX|T GAUGE ERROR GAUGE ERROR ggg ASD(N-l)- 402 ASDUW} 405 410 TRY r TOTAL 2 ENTRY GAUGE STAND(N-I) I ERROR DETERMINED cORREcT ION 408 AT sTAND (N) sTANDIN-I) CORRECTION FOR 7 FEEDBACK ERROR EAIENTED 1 51974 3,841 123 SHEET t 0? 4 wmmmooq r ROLLING MILL GAUGE CONTROL METHOD AND APPARATUS INCLUDING ENTRY GAUGE CORRECTION CROSS REFERENCE TO RELATED APPLICATIONS Reference is made to the following previously filed and related patent application which is assigned to the present assignee:

Ser. No. 215,743, filed Jan. 6, 1972 and entitled Gauge Control Method and Apparatus For Metal Rolling Mills and filed by A. W. Smith and R. Q. Fox.

Reference is made to the following concurrently filed and related patent applications which are assigned to the present assignee:

Ser. No. 303,723, filed Nov. 6, 1972 entitled Rolling Mill Gauge Control Method and Apparatus Including Temperature and Hardness Correction and filed by A. W. Smith. Ser. No. 303,725, filed Nov. 6, 1972 entitled Rolling Mill Gauge Control Method and Apparatus Including Speed Correction and filed by R. Q. Fox.

Ser. No. 303,724, filed Nov. 6, 1972 entitled Rolling Mill Gauge Control Method and Apparatus Including X-Ray Correction and filed by R. Q. Fox.

Ser. No. 303,722, filed Nov. 6, 1972 entitled Rolling Mill Gauge Control Method and Apparatus Including Feedback Correction and filed by R. QgFox and D. J. Emberg.

Ser. No. 303,726, filed Nov. 6, 1972 entitled Rolling Mill Gauge Control Method and Apparatus Including Plasticity Determination and filed by R. Q. Fox.

BACKGROUND OF THE INVENTION The present invention relates to workpiece strip metal tandem rolling mills and more particularly to roll force gauge control systems and methods used in operating such rolling mills.

In the operation of a metal or steel reversing or taridem rolling mill, the unloaded roll opening and the speed at each tandem mill stand or for each reversing mill pass are set up to produce successive workpiece strip or plate reductions resulting in work product at the desired gauge. Generally, the loaded roll opening at a stand equals the stand delivery gauge or thickness on the basis of the usual assumption that there is little or no elastic work-piece recovery.

Since the operator provided initial roll opening setup conditions, or the initial roll opening settings-provided by an associated digital computer control system operative with model equation information to calculate the setup screwdown schedules for the respective stands of the rolling mill, can be in error and since in any event of the rolling mill art, and particularly the steel rolling mill an, a stand gauge control system is normally used fora reversing mill stand and for predetermined stands in tandem rolling mills.

The well known gaugemeter or roll force system has been widely used to produce stand gauge control in metal rolling mills and particularly in tandem hot steel strip rolling mills and reversing plate mills where experience has demonstrated that roll force control is particularly effective. Earlier publications and patents such as an article entitled Installation and Operating Experience with Computer and Programmed Mill Controls by M. D. McMahon and M. A. Davis in the I963 Iron and Steel Engineer Year Book at pages 726 to 733, an article entitled Automatic Gage Control for Modern Hot Strip Mills by .l. W. Wallace in the December I967 Iron and Steel Engineer at pages to 86, US. Pat. No. 3,561,237 issued to Eggers et al. and US. Pat. No. 2,726,541 issued to R. B. Sims describe the theory upon which operation of the roll force and related gauge control systems are based. Attention is also called to US. Pat. Nos. 3,568,637, 3,574,279, 3,574,280 and 3,600,920. issued to A. W. Smith, which relate to roll force automatic gauge control systems. In referencing prior art publications or patents as background herein, no representation is made that the cited subject matter is the best teaching prior art.

Briefly, the roll force gauge control system uses Hookes law in controlling the screwdown position at a rolling stand, ie the loaded roll opening under workpiece rolling conditions equals the unloaded roll opening or screwdown position plus the mill stand spring stretch caused by the separating force applied to the rolls by the workpiece. To embody this rolling principle in the roll force gauge'control system, a load cell or other force detector measures the roll separating force at each controlled roll stand and the screwdown position is controlled to balance roll force changes from a reference value and thereby hold the loaded roll opening at a substantially constant value. Hot strip mill automatic gauge control (AGC) including evaluation of roll force feedback information involves the combination of a number of process variables, such as roll force, screw position, and mill spring which are all used to evaluate the gaugeof the strip as it is worked in each stand. In addition, an X-ray gauge is used on the strip as it passes out of the last stand to evaluate the absolute strip gauge produced.

The two gauge error detection systems that are commonly used are the X-ray and roll force. X-ray gauges can be placed between each stand, but they are expensive, difficult to maintain, and can detect errors only as the strip passes between stands. The roll force error detection system is much less expensive, and can be more easily implemented in relation tothe operation of all stands, to detect errors in gauge as the strip passes between the rolls of a particular roll stand, providing immediate evaluation of desired corrections to the roll openings; The roll force system, however, provides onlya relative evaluation of the gauge, since itmeasuresthe amount of gauge deviation from a reference gauge, such asthe' gauge at the head end of the strip.

A practical combination of the twosysternsuses'rollforce feedback to calculate fast corrections to fluctua- I tions in gauge, and an X-ray gauge to evaluate the absolute gauge of the strip coming out of the last stand. The

' fast corrections are calculated from the rollforce feedback, the stand screwdown position, and the modulus of elasticity of the rolling stand. The slower X-ray gauge evaluation caculates simultaneous corrections to several stands, so that'the absolute value of the gauge may be brought to the desired value.

The output of both of these systems is a change in the position references supplied to the screwdowns of selected roll stands.

The following well known formula expresses the basic roll force gauge control relationship:

where:

h loaded roll opening (workpiece delivery gauge or thickness) SD unloaded roll opening (screwdown position) K stand mill spring constant F stand roll separating force. Typically, the roll force gauge control system is an analog arrangement including analog comparison and amplification circuitry which responds to roll force and screwdown position signals to control the screwdown position and hold the following equality:

where:

AF measured change in roll force from an initial force ASD controlled change in screwdown position from an initial screwdown position.

After the unloaded roll opening setup and the stand speed setup are determined by the mill operator for a particular workpiece pass or series of passes, the rolling operation is begun and the screwdowns are controlled to regulate the workpiece delivery gauge from the reversing mill stand or from each roll force controlled tandem mill stand. By satisfying Equation (2), and the assumptions implicit in Equation (1), the loaded 'roll opening h in Equation (1) is maintained constant or nearly constant.

As the head end of the workpiece strip enters each roll standof the mill, the lock-on screwdown position LOSD and the lock-on roll separating force LOF are measured to establish what strip delivery gauge G should be maintained out of that roll stand. As the strip rolling operation proceeds, the roll stand separating force F and the roll stand screwdown position value SD are monitored periodically and any undesired change in roll separating force is detected and compensated for by a corresponding correction change in screwdown position. The lock-on gauge LOG is equal to the lockon screwdown LOSD plus the lock-on force LOF multiplied by the mill stand spring modulus K. The workpiece strip delivery gauge Gleaving the 'roll stand at any time during the rolling operation is in'accordance with above equation (I) and is equal to the unloaded screwdown position SD plus the roll separating force F multiplied by the mill spring modulus K. The roll force determined gauge error GE in relation to a particular roll stand is derived by subtracting the lock-on gauge LOG from the present delivery gauge G. The following Equations 3, 4 and set forth these relationships.

LOG LOSD K*LOF 3) G SD K*F (4) GE G LOG SD LOSD (F LOF)*K 5 To provide steady state gauge error correction, the well known X-ray monitor gauge control system is usually employed to produce screwdown offset for the roll force control. In the monitor system, an X-ray or other radiation gauge sensing device is placed at one or more predetermined process points, and usually at least at a process point following the delivery end after the last roll stand of the mill, in order to sense actual delivery gauge after a workpiece transport delay from the point in time at which the actual delivery gauge is produced at the preceding stand or stands. The monitor system compares the actual delivery gauge with the desired delivery gauge and develops an X-ray gauge error as an analog feedback control signal to adjust the operation of the reversing mill roll force gauge control system or one or more predetermined tandem mill stand roll force gauge control systems to supply desired steady state mill delivery gauge. In this manner, the conventional monitor system provides for transport delayed correction of steady state gauge errors which are caused or-which are tending to be caused by a single mill variable or by a combination of mill variables.

In operator controlled mills, some steady state gauge correcting operations can eventually be taken off the monitor system by screwdown recalibration, and the like, between similar workpiece passes if steady state gauge error tends to exist along the entire workpiece and persists from workpiece to workpiece. In this manner, some reduction is achieved in the length of off gauge workpiece material otherwise associated with monitor transport delay. Similarly, corrective monitor system operation caused by head end gauge errors can be reduced by changes in the operator or associated computer control system provided setup from workpiece to workpiece.

A background general teaching of stored program digital computer control system operation is set forth in a book entitled Electronic Digital Systems by R. K. Richards and published in 1966 by John Wiley and Sons.

An additional detailed description of computer programming techniques in relation to the control of metal rolling mills can be found in an article in the Iron and Steel Engineer Yearbook for 1966 at pages 328 through 334 entitled Computer Program Organization for an Automatically Controlled Rolling Mill by John S. Deliyannides and A. H. Green, and in another article in the Westinghouse Engineer for January 1965 at pages 13 through 19 and entited Programming for Process Control by P. E. Lego. i i

A programmed digital computer system can be employed to make'the gauge error correction screwdown movement determinations as'well as to perform other mill control functions. The computer employs a programming system" including an automatic roll force gauge control program or AGCprogram which is executed at predetermined periodic intervals to calculate the desired screwdown movement required at each roll force gauge controlled stand for gauge error correction including that stemming from roll force error detection at that stand.

SUMMARY OF THE INVENTION In accordance with the broad principle of the present invention, a system and method for controlling gauge in a metal rolling mill employs means for detecting at least one error condition representing gauge error in the workpiece delivered from a given roll stand, which roll stand is not permitted to make a screwdown adjustment to correct this gauge error, and means for controlling the screwdown position of at least one previous rolling stand of the mill for providing a predetermined correction to the entry gauge in relation to that given roll stand and this detected gauge error.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a schematic diagram of a tandem hot steel strip rolling mill and an automatic gauge control system arranged for operation in accordance with the present invention;

FIG. 2 illustrates the typical mill spring curve and workpiece reduction curve for a given rolling mill stand and the operation of that roll stand for reducing the gauge of a workpiece passed through the roll stand;

FIG. 3 illustrates, in relation to the mill spring curve and the workpiece reduction curve, the effect of a correction made to the screwdown position setting for changing the unloaded roll opening of a roll stand to provide a desired change in the workpiece gauge 'delivered from that roll stand.

FIG. 4 shows an illustrative correction of a determined gauge error for a roll stand by changing the entry gauge of the workpiece before it is passed through that roll stand.

FIG. 5 shows an adjustmentmade in the screwdown position of previous stand N-l as required to provide a desired change in the entry gauge of the workpiece in relation to the operation of the subsequent roll stand N.

DESCRIPTION OF THE GAUGE CONTROL SYSTEM AND ITS OPERATION There is shown in FIG. 1 a tandem hot strip steel finishing mill 11 operated with improved, gauge control performance by a process control system 13 in accor- In the reduction rolling process, the successive stands operate at successively higher speeds to maintain proper workpiece mass flow. Each stand produces a predetermined reduction or draft such that the total mill draft reduces the entry bar to strip with the desired gauge or thickness.

. Each stand is conventionally provided with a pair of backup rolls l9 and 21 and a pair of work rolls'23 and 25 between which the workpiece 15 is passed. A large DC drive motor 27 is controllably energized at each stand to drive the corresponding work rolls at a controlled speed.

As previously described, the sum of the unloaded work roll opening and the mill stretch substantially defines theworkpiece gauge delivered from any particular stand in accordance with I-Iookes law. To vary the unloaded work roll opening at each stand, a pair of screwdown motors 29 (only one shown at each stand) position respective screwdowns 31 (only one shown at each stand) which clamp against opposite ends of the backup rolls and thereby apply pressure to the work rolls. Normally, the two screwdowns 31 at a particular stand would be in identical positions, but they can be dance with the principles of the invention. Generally,

' coiler 17. The entry bar would be of known steel grade class and it typically would have a known input gauge or thickness of about 1 inch and'a width within some limited range such as 20 inches to 80 inches. The delivered strip would usually have approximately the same width and a thickness based upon the production order for which it is intended.

located in different positions for strip guidance during threading, forflatness or other strip shape control purposes or possibly for otherpurposes. g

A conventional screwdown position detector or encoder 33 provides an electrical signal representation of screwdown position at each stand. To provide an absolute correspondence between the screwdown position and the unloaded roll opening between theassociated work rolls, a screwdown position detection system which includes the screwdown position detector 33 can be provided and calibrated from time to time.

Roll force detection is provided at each of predetermined stands by a conventional load cell 35 which generates an electrical analog signal in accordance with the stand roll force. At the very least, each roll force controlled stand is provided with a load cell 35 and in many cases stands without roll force gauge control would also be equipped with load cells. The number of stands to which roll force gauge control isapplied is predetermined during the mill design in accordance with cost-performance standards, and increasingly there is a tendency to apply roll force gauge control to all of the stands in a tandem hot strip steel mill. In the present case, a roll force gauge control system is assumed to be employed at each of the stands.

Conventional motorized sideguards 37 are located at predetermined points along the mill length. The sideguards 37 areoperated during'mill setup on the basis of the widths of the upcoming workpiece 15 thereby defining the sides of the workpiece travel path for guidance purposes.

The process control system 13 provides automatic control for the operation of the tandem mill [1 as well as desired control for associated production processes (not indicated) such as the operation-of a roughing mill. The process control system 13 can include a programmedprocess contol digital computer system which is interfaced with the various mill sensors and the various mill control devices to provide control over many of the various functions involved in operating the tan dem mill 1 1. According to user preference, the control system 13, can also include conventional manual and- /or automatic analog controls for selected process control functions.

On the basis of these considerations, automatic gauge control system 39 can include a digital computer system operative to provide the finishing mill on-line roll force gauge control function, such as a Prodac 2000 (P2000) sold by Westinghouse Electric Corporation. A descriptive book entitled Prodac 2000 Computer Systems Reference Manual has been published in I970 by Westinghouse Electric Corporation and made available for the purpose of describing in greater detail this computer system and its operation.

There is disclosed in the above-referenced previously filed patent application Ser. No. 215,743 the logic flow chart of an illustrative automatic gauge control suitable for operation in conjunction with the entry gauge correction operation of the present invention. It should be readily understood by persons skilled in this art that the present invention is also suitable for operation with other presently well known automatic gauge control systems for controlling the delivery gauge of a workpiece strip passed through at least two stands of a rolling mill.

The digital computer processor can be associated with well known predetermined input systems typically including a conventional contact closure input system which scans contact or other signals representing the status of various process conditions, a conventional analog input system which scans and converts process analog signals, and operator controlled and other information input devices and systems 41 such as paper tape teletypewriter and dial input systems. It is noted that the information input devices 41 are generally indicated by a single block in FIG. 1 although different input devices can and typically would be associated with the control system. Various kinds of information are entered into the control system through the input devices 41 including, for example, desired strip delivery gauge and temperature, strip entry gauge and width and temperature (by entry detectors if desired), grade of steel being rolled, plasticity tables, hardware oriented programs and control programs for the programming system, and so forth. The principal control action outputs from the automatic gauge control or AGC system include screwdown poisitioning reference commands which are applied to respective screwdown positioning controls 55 for operating the screwdown motors 29 for screw movement, and speed control signals which are applied to the respective speed and tension control system 53 to cause a change in drive speed to compensate for a change in thickness being made by a screwdown movement.

Display and printout devices 51 such as numeral display, tape punch, and teletypewriter systems can also be provided to keep the mill operator generally informed about the mill operation and in order to signal the operator regarding an event or alarm condition which may require some action on his part. The printout devices are also used to log mill data according to computer log program direction.

Generally, the AGC system uses Hookes law to determine the total amount of screwdown movement required at each roll force controlled stand at the calculating point in time for roll force and gauge error correction, i.e. for loadedroll opening and stand delivery gauge correction to the desired value. The calculation defines the total change in the unloaded roll opening required to offset the gauge error causing condition.

During rolling operation, the on line gauge control system operates the stands to produce strip product having desired gauge and proper shape. i.e. flat with slight crown. On line gauge control is produced by the roll force gauge control loops at the stands and the previously noted X-ray monitor gauge control system.

In the monitor system, the X-ray gauge 47- produces the X-ray gauge error or deviation signal which indicates the difference between actual strip delivery thickness and desired or target strip delivery thickness. In other cases, it may be desirable to employ an absolute thickness measurement X-ray gauge signal to form a basis for monitor control actions or, more generally, for screwdown offset control actions.

To effect on line gauge control in the closed loops, the AGC system operates at predetermined time periods such as every 2/ 10 second with the screwdown position detector and load cell provided signals from each stand as well as the X-ray gauge error signal to determine the respective stand screwdown adjustment control actions required for producing desired strip delivery gauge.

In FIG. 2, linear approximations of the roll stand characteristic curves are shown to illustrate the application of Hookes law to a rolling mill stand and to illustrate the basis upon which the on line gauge control system provides improved gauge control, accuracy and stability and other operating benefits. A mill modulus characteristic or mill spring curve defines the separation between a pair of workpiece reducing mill stand work rolls as a function of separating force and as a function of screwdown position. The slope of the mill spring curve 100 is the well known mill spring modulus or constant K which is subject to variation as well known to persons skilled in this art. When a correct screwdown calibration is known and the screwdowns are positioned such that the empty work rolls are just facing, the unloaded screwdown zero position is defined. The workpiece deformation characteristic or reduction curve 102 is shown. The entry gauge H of the workpiece passed through the roll stand is reduced to the indicated delivery gauge H as defined by the intersection of the mill spring curve 100 and the product reduction curve 102 to establish the stand roll force required for the indicated operation. The unloaded roll opening, sometimes called the screwdown because of the screw and nut system used for adjusting the roll opening, is the gauge that would be delivered if there was no roll separating force. As the force increases with a constant roll opening, the delivery gauge increases, since the mill deflects as shown by the mill spring curve 100. If no force was exerted on the product being rolled, the gauge would not be reduced and the delivery gauge would be equal to the entry gaugeJWhen the roll force increases,'the product is'pla'stically deformed and the delivery gauge decreases; The slope' of the mill spring characteristic line is called the mill modulus (K) and the slope of the product reduction characteristic is called the product plasticity (P). The delivery gauge is determined by the equilibrium point at which the force exerted by the mill is equal to the force required to deform the product. Changes in entry gauge and product hardness result in a change-in roll force and delivery gauge. The automatic gauge control moves the screwdown to correct for these gauge changes. The main advantage of the roll-force gauge control system is its ability to detect changes in gauge the instant they take place, as the product is being rolled in the stand. A shift in delivery thickness can be caused by a change in entry thickness or a change in hardness (usually caused by a change in temperature). This change in delivery gauge is immediately detected by monitoring the roll separating force of the roll stand.

When the screwdowns are opened (positive movement) the unloaded roll opening increases as reflected by a change to the right in the graphical location of the mill spring curve 100 such that the theoretical spring curve intersect equals the new unloaded roll opening. With screwdown closing, the mill spring curve is shifted to the left in a similar manner.

At any particular screwdown position and with correct screwdown calibration, the stand workpiece delivery gauge I-I equals the unloaded roll opening as defined by the screwdown position SDREF plus the mill stretch (F*K) caused by the workpiece. If the screwdown calibration is incorrect, i.e., if the number assigned to the theoretical roll facing screwdown position is something other than zero because of roll crown wear or other causes, the stand workpiece delivery gaugeI-I then equals the unloaded roll opening plus the mill stretch, plus or minus the calibration drift.

The amount of mill stretch depends on the product deformation characteristicor reduction curve 102 for the workpiece. As shown in FIG. 2, the reduction curve 102 for a strip of predetermined width represents the amount of force Frequired to reduce the workpiece from the stand entry gauge (height) H The workpiece plasticity P is the slope of the curve 102, and the curve 102 is shown as being linear although a small amount of nonlinearity would normally exist.

Desired workpiece delivery gauge H is produced since the amount of force F required to reduce the workpiece from H to H to equal to the amount of roll separating force required to stretch the rolls to a loaded roll opening H i.e. the intersection of the mill spring curve 100 at an initial screwdown opening SDREF indicated by mill spring curve 100 and the workpiece reduction curve 102 lies at the desired gauge value H As shown in FIG. 3, if the actual stand present gauge Hx is not the same as the desired gauge I-I there is a gauge error GE to be corrected. This condition can be corrected by changing the provided screwdown position reference SDREF to the stand, such that a new mill spring curve 104 becomes operative to result in the desired gauge I-I being delivered from the roll stand and the gauge error GE is now removed.

However, it may be desired that the screwdown position reference provided to this roll stand in relation to the illustration of FIG. 3, for example, roll stand N, not be changedfThe roll opening of roll stand'N might already be moved to its predetermined limit of adjustment or the operator for some different reason may not desire to further adjust the roll opening of stand N. As

shown-in FIG. 4', theentry gauge H U) of the work-- The workpiece exit gauge leaving a previous roll stand (N-l) is equal to the workpiece entry gauge ad vanced to the next succeeding roll stand (N) since no reduction has been made in the workpiece gauge.

Exit gauge (N-l) entry gauge (N) It is known in accordance with the teachings of above referenced U.S. Pat. No. 3,561,237 that the required corrective screwdown adjustment ASD to correct a stand delivery gauge error GE is equal to the product of that gauge error GE times the sum of the ratio of the workpiece plasticity P for that same stand with the mill spring modulus K for that same stand and one, as follows in relation to stand (N):

ASD(N) exit GE(N)= [P(N In the teachings of the latter patent, the mill spring .constant K(N) was defined in terms of millions of From a logical extension of the above equation (6), it can be established that the exit gauge error GE(N) leaving stand (N-l) is equal to the entry gauge error entering the succeeding roll stand N, since no reduction is made in the workpiece thereby:

Exit gauge error (N-l) entry gauge error (N) The total stand (N-l) gauge error GE(N- l) that it is desired be corrected by screwdown adjustment at stand N-l is equal to the roll force system determined gauge error at stand (N-l) plus the desired tobe corrected feedback gauge error from stand N that is determined at stand N by operation of the stand N roll force systern.

TOTAL GE(N- l) roll force exit gauge error GE(N- v l) feedback entry gauge error GE(N) (ID) The stand (N-l) exit gauge error determined by the A roll force system at stand (N-l) is established by the relationship of above equation(5) as follows:

The stand 'N exit gauge errordetermined by the roll force system at stand (N) is established by the relationship of above equation (5) as follows:

GE(N) SD(N) LOSD(N) [P(N) J Since the stand N screwdown mechanism is now assumed to be in position movement limits, such that no screwdown position adjustment is to be made at stand N, it is desired that this stand N exit gauge error be corrected by changing the entry gauge of the workpiece strip before it enters stand N, which change is effected by a correlated screwdown adjustment of previous stand (N-l) such that the exit gauge from stand (N-l) is now changed in accordance with the desired entry gauge correction for stand N, which is applied as a feedback correction from stand N.

The entry gauge correction of stand N that is.determined by operation of stand N roll force systemis the product of the measured stand N exit gauge error GE(N) times the quantity K(N) divided by P(N) plus one, as follows:

Entry gauge error GE(N) exit GE(N)=[P(N)/K(N) I 13) The required feedback screwdown correction needed at previous stand (NJ) to remove the stand N entry gauge error, since the stand N is not permitted to make the screwdown adjustment for this purpose, is established by combining above equations (8) and (13) as follows:

The total screwdown correction needed at stand (N-l) for correcting both the exit gauge error measured at stand (N-l) and the entry gauge error measured at stand N is determined as follows:

I mined at stand (N-l) by the roll force system of stand (N-l) is desired to be corrected by suitable screwdown adjustment at stand (N-l). The entry gauge error at stand N that is determined at stand N is the equivalent of an exit gauge error leaving stand (N-l) and determined at stand N as above set forth.

As shown in FIG. 5, to correct the exit gauge error GE detected in the operation of stand N by changing the entry gauge to stand N, this requires that the delivery gauge-from stand (N 1) be changed by making the detennined screwdown position correction in relation to stand (N-l The required feedback screwdown correction ASD(N-l) is based uponthe exit gauge error GE of stand N,-the predetermined mill spring modulus I(( N1) of stand (N-l) and the known product plasticity P( N-l) at stand (N-l The required screwdown position correction for stand (N-l) is provided and is determined in accordance with the relationship of equation The timing for the making of successive screwdown position feedback corrections in relation to the operation of the previous operating stand (N-l) should be such that when each such screwdown position feedback correction is actually made to result in the desired correction to the entry gauge being supplied to the succeeding stand N, no further such feedback correction is to be made until the corrected workpiece portion has moved from the previous stand (NJ) to the error detecting stand N. In addition, this same entry gauge correction equation can be utilized when a gauge error in relation to a given roll stand is detected, for example by an X-ray gauge positioned after that given roll stand such as the last roll stand and screwdown position feedback corrections are then applied to one or more of the preceding stands of the rolling mill.

In reference to FIG. 6, in general the workpiece strip gauge error delivered by a given stand, and as determined by the sensed operational variables at that same stand, is in accordance with the roll force system relationship shown in above equation l2). The exit gauge error leaving stand (N), for example, equals the sum of a first quantity, which is the difference between the presently measured screwdown position SD(N) and the initiallock on screwdown position LOSD(N), and a secondquantity, which is the determined mill spring modulus K(N) times the difference between the presently measured roll separation force F(N) and the initial lock on roll force LOF(N).

DESCRIPTION OF EMBODIMENT OF PRESENT INVENTION In reference to FIG. 7, there is shown a portion of a tandem rolling mill including a roll stand (N-l), followed by a roll stand (N), with the workpiece strip 15 moving in the direction indicated'by the arrow. At block 400 there is determined the exit gauge error leaving stand (N-l) in relation to the operational variables sensed at stand (N-l This determination utilizes above equation (11) for this purpose. At block 402 there is detennined the required stand (N-l) 'screw down position correction to remove this same exit gauge error leaving stand (N-l and this utilizes above equation (8) for this purpose as set forth in relation to stand (N-l). At block 404 there is determined the exit gauge error leaving stand (N) in relation to the opera- I tional variables sensed at stand (N), and this determination utilizes above equation (12) for this purpose. At block 405 there is determined the required stand (N) screwdon correction to remove this exit gauge error, however, it is desired that the stand (N) screwdown position not be changed for this purpose. At block 406 there is determined the entry gauge error entering stand (N) in relation to the operational variables sensed at stand (N), and this utilizes above equation (13) for this purpose. At block 408 there is determined the stand (N-l) screwdown position correction requir'ed to remove this stand N entry gauge error, and this utilizes above equation (14-) for this purpose. At block 4l0there is determined the totalstand (N-l) screw-down position correction needed. to remove both the exit gaugeerror at stand (N-l) as well as the entry gauge errorv at stand (N), and this utilizes above equation (15) for this purpose. I

In the event that the nextprevious roll stand N-l is also in a limited screwdown position condition of operation, it may be desired to adjust in this manner the screwdown position of the roll stand N-2 in relation to the exit gauge error detected by the screwdown position limited roll stand N.

The absolute physical position of the screwdown adjustment mechanism for any given roll stand is limited. Before a screw position is corrected, its absolute position is checked to see if it is presently at its minimum bound or maximum bound. If the screw is positioned to one of its limits, and the desired gauge error correction will cause it to exceed that limit, a feedback correction is made to the entry gauge of this stand. The following general operational conditions should be met before a feed-back correction is to made in relation to the operation of Stand N:

1. Stand N is not the first roll stand.

2. The load cell for previous stand (N-l) is not on or stand N screw adjustment is out of limits,

3. The load cell for stand N is on, and

4. A feedback entry gauge correction in relation to stand N is not already in progress.

If all these conditions are met, the feedback program can be entered. If one of these conditions is not true, the feedback program is skipped.

In relation to the here provided entry gauge error correction feedback program in FIG. 8 there is shown a flow chart to illustrate the operation of this program, which runs periodically every time an automatic gauge control operation takes place to establish the desired screwdown position adjustment at each of the roll stands in the rolling mill. At step 270, if this is the first stand then no error correction can be physically fed back to an earlier stand so the program ends. At step 272 the preceding stand (N-l) is addressed, with the present stand being stand N. At step 274 the desired flags in relation to operator established screwdown movement limits and stand calibration are picked up, and it is now desired to check why the feedback program was entered. At step 280 a check is made to see if the stand (N-l) screw positioning mechanism was properly calibrated and if so the feedback correction is calculated at step 278. In general, the feedback program can be enetered if stand N went into screw position limits as well as if the load cell of-stand (Nl) was faulty. At step 282, the stand address indexing is set up and the program operation ends.

The typical AGC control program is written as a loop operation such that one set of coding processes all of the roll stands, and every time the program operates through the loop a calculation is made to determine the desired screwdown adjustment for each of the roll stands in relation to the gauge error and the feedback correction when appropriate.

In FIG. 9 there is shown a more detailed feedback correction control program in accordance with the present invention. The program can feedback agau'ge error correction not only to one previous stand, but can feed back such a correction two stands if desired. This I gives the program additional power under the situation where stand N is in limits, and stand (N-l) 'is in limits, and it .is desired to make a correction at stand (N-2).

' The provided logic operation selects which previous stand to feed the determined correction back to. If the load cell of previous stand (N-l) is broken and the load cell of stand N is not broken, that is a legitimate reason correction screw position movement, such that it is desired to feed back a correction to the previous stand -l).

The Feedback program makes the following three checks on previous stand (N-l):

1. Stand (NJ) is not 0.

2. The screw for stand (N-l) is not at the same position limit as stand N, i.e. if stand N is at its up limit, stand (N-l) is not at its up limit.

3. The screws for stand (N-l) are in calibration. If all these three checks are true, a feedback correction to the exit gauge of stand (N-l) can be made. The feedback in progress flag for stand N is set and a time delay is initiated, so that no further feedback gauge error correction can be made to stand (N-l) until the present feedback correction reaches stand N. The time delay equals the determined transport time between stand (N-l) and stand N. If one of the above three subroutine checks fails for stand (N-l), the entire checking procedure is repeated, with stand (N-2) substituted for stand (N-l). If one of the three checks also fails for stand (N-2), the feedback routine is exited without making any corrections.

The feedback routine shown in FIG. 9 can operate back two previous stands if it is necessary, so the first thing to determine is if stand N is the first stand. If this is stand one at step 900 then stand (N-l) is not greater than zero and this ends the program since there is no previous stand to feed back 'a correction to; If this is not stand 1, then stand (N-l) is greater than zero at step 900. The program then advances to address stand (N-l) at step 902 and checks the limit flags and the calibrat io'n flags at step 904 for stand (N-l) to determine which stand to operate on. The program at step 906 detennines that stand (N-Z) is not stand 0, and if it is not, at step 908 the program addresses stand (N-2) and picks up the limit and calibration flags at step 910 for that stand. At step 912 a comparison is made of the flags for stands N, (NJ) and (N-2) trying to find the proper stand to feed a correction back to. At this point the feedback routine has been entered, but it. is not known if it was entered because the roll force, sensing was faulty at stand (N-l) or this stand N was in limits. By assuming that this stand N is in limit, a determination is made at step 914 tosee if stands N and (N-l) are at the same limit. If they are, then it is necessary to go back to stand (N-2) to make the feedback correction.

If they are not, at step 916 a check is made to see if the stand (N-l) is calibrated, since there is not a desire to feed back to a'dead stand. lf stand (N-l) is not calibrated, a check is made at step 918 to" see if stands N and (N-2) are at the same limit condition. If not, at step 920 a check is made to see if stand (N-2) is calibrated. If stand (N-"l )is calibrated at step 916, then at step 922 the address of that stand is saved and the program goes to the transport time delay calculation at step 924. Similarly, if stand (N-'2) is calibrated at step 920, the address of that stand is saved at step 926 and the program goes to step 924 since there is a possibility of doing a feedback correction operation. The program has selected a previous stand to feed a gauge'error correction back to and is now going to calculate the feedback correction itself. At step 924 there is calculated a transport time delay for'a segment of'workpiece strip to pass from stand N-l to stand N. The feedback correction operation is not run continuously, but instead is run once every transport time delay. A correction is made at stand (N-l) and then the program waits until the correction has been received at the detection stand N so that the control operation is stable and not running away. The transport time delay calculation depends on whether the feedback correction is to be made at stand (N-l) or with stand (N-2). The flow chart includes a check at step 928 to see whether an additional time delay is to be calculated at step 930 from stand (N-2) to (N-l) in order to get the true transport time delay from where the correction is to be made. These calculations are determined in relation to the known strip speed and known stand distance to establish the travel or time delay. At step 932 the calculation is made of the actual feedback correction, which is the calculation in accordance with FIG. 7. The feedback correction value is stored in memory at step 934 when the automatic gauge control operation for the corrected roll stand either (N-l) or (N-2), using the relationships of above equation (5) calculates the exit gauge error for the corrected roll stand and above equation (8) determines the stand screwdown adjustment needed to correct the exit gauge error detected at that same roll stand, this additional feedback correction is added and also applied at the same time. At step 936 the feedback in progress flag is set to indicate that a correction is being made and a time delay is needed for future calculations. At step 938 a time delay operation is done by setting up a timer to prevent reentering the feedback routine until the required transport time is elapsed.

In the operation of an actual hot strip rolling mill to which the present control system was applied, the workstrip entry gauge into the first stand was in the order of 30 to 36 millimeters and the exit gauge leaving the last stand was in the order of 1.50 to 12 millimeters. The screwdown movement position limits for the respective roll stands were established as follows in terms of position encoder units, with the relationship being 80 encoder units per millimeter:

Positive Limit Negative Limit Stand One Stand Two Stand Three Stand Four Stand Five Stand Six In the Appendix there is included an instruction program listing that has been prepared to control the roll force automatic gauge control operation of a tandem rolling mill in accordance with the here disclosed control system and method. The instruction program listing is written inthe machine language of the PRODAC P2000 digital computer system, which is sold by Westinghouse Electric Corporation for real time process control computer applications. Many of these digital computer systems have already been supplied to customers, including customer instruction books and descriptive documentation to explain to persons skilled in this art the operation of the hardware logic and the executive software of this digital computer system. This instruction program listing is included to provide an illustration of one suitable embodiment of the present control system and method that has actually been prepared. This instruction program listing at the present time has been partially debugged through the course of practical operation for the real time automatic gauge control of a tandem rolling mill, but it is understood and well known by persons skilled in this art that most real time process control application programs contain some bugs or minor errors, and it is within the routine skill of such persons and takes varying periods of actual operation time to identify and correct the more important of these bugs.

A person skilled in the art of writing computer instruction program listings, particularly for an invention such as the present roll force automatic gauge control system and method for a tandem rolling mill must generally go through the following determinative steps.

Step One Study the workpiece rolling mill and its operation to be controlled, and then establish the desired control system and method concepts.

Step Two Develop an understanding of the control system logic analysis, regarding both hardware and software.

Step Three Prepare the system flowcharts and/or the more detailed programmerss flowcharts.

Step Four Prepare the actual computer instruction program listings from the flowcharts.

What we claim is:

l. A gauge control system for a rolling mill having at least a first roll stand followed by a second roll stand operative to reduce thegauge of a workpiece passed through said rolling mill, said system comprising:

means for determining the delivery gauge error of said workpiece leaving said second roll stand in accordance with a first predetermined relationship including the change in the screwdown position of said second roll stand and the change in the roll force of said second roll stand,

means for determining the required adjustment of said first roll stand for correcting said delivery gauge error in accordance with a second predetermined relationship including the mill spring modulus of said second roll stand and the workpiece plasticity in relation to said second roll stand, and

means for controlling the roll opening of said first roll stand during the passage of said workpiece in accordance with said required adjustment.

2. The gauge control system of claim 1, with said 7 GE(N): [SD(N) Losmm [F(N) LOF(N)]=K(N) v where GE(N) is the delivery gauge error leavingsaid second roll stand, 1

where [SD(N) LOSD(N)] is the change in down position of. said second roll stand,

screwwhere [F(N) LOF(N)] is the change in roll force of said second roll stand, and where K(N) is the mill spring modulus of said second roll stand.

4. The gauge control system of claim 1, with said second predetermined relationship being in accordance with the following equation:

where ASD(N-l) is the required adjustment of said first roll stand,

where GE(N) is the delivery gauge error leaving said second roll stand,

where P(N) is the workpiece plasticity in relation to said second roll stand,

where K(N) is the mill spring modulus of said second roll stand,

where K(N-l) is the mill spring modulus of said first roll stand, and

where P(N-l) is the workpiece plasticity in relation to said first roll stand. 5. The gauge control system of claim 1, with said second roll stand being in a condition of limited operation for controlling said gauge error such that it is desired for said first roll stand to correct said gauge error.

6. A gauge control system for a rolling mill having at least a first roll stand followed by a second roll stand and being operative to reduce the gauge of a workpiece passed through said first and second roll stands, said system comprising:

means for determining the second stand delivery gauge error of said workpiece leaving said second roll stand in relation to the roll force of that second roll stand and with said second roll stand being in a condition of roll opening position adjustment limit, means operative in accordance with a predetermined relationship including said second stand delivery gauge error for determining the required adjustment of said first roll stand to correct said second stand delivery gauge error by the passage of said workpiece through said first roll stand, and

means for providing said required adjustment of said first roll stand during the passage of said workpiece through said first roll stand.

7. The gauge control system of claim 6, with said determination of said required adjustment ASD(N-l) being in accordance with the following relationship:

where GE(N) is the second stand delivery gauge error,

where P(N) is the workpiece plasticity in relation to the second roll stand,

where K(N) is the mill spring modulus of the second roll stand,

where K(N-l) is the mill spring modulus of the first roll stand, and

where P(N-l) is the workpiece plasticity in relation to the first roll stand.

8. A method of controlling the gauge of a workpiece passed through a rolling mill having at least a previous roll stand followed by a later roll stand for reducing said gauge, the steps of said method comprising:

determining the delivery gauge error of said workpiece leaving said later roll stand during the passage of said workpiece through that later roll stand,

determining the required correction of the operation of said previousroll stand for correcting said delivery gauge error during the passage of said workpiece through said previous roll stand in accordance with a predetermined relationship including the mill spring modulus and the workpiece plasticity in relation to said later roll stand, and

adjusting the operation of said previous roll stand inaccordance with said required correction. 9. The method of claim 8, with said predetermined relationship being as follows:

where ASD(N-l) is the required correction of the operation of the previous roll stand,

where P( N) is the workpiece plasticity in relation to said later roll stand,

where K(N) is the mill spring modulus of said later roll stand,

where l((N-l) is the mill spring modulus of said previous roll stand, and

where P(N-l) is the workpiece plasticity in relation to said previous roll stand. 

1. A gauge control system for a rolling mill having at least a first roll stand followed by a second roll stand operative to reduce the gauge of a workpiece passed through said rolling mill, said system comprising: means for determining the delivery gauge error of said workpiece leaving said second roll stand in accordance with a first predetermined relationship including the change in the screwdown position of said second roll stand and the change in the roll force of said second roll stand, means for determining the required adjustment of said first roll stand for correcting said delivery gauge error in accordance with a second predetermined relationship including the mill spring modulus of said second roll stand and the workpiece plasticity in relation to said second roll stand, and means for controlling the roll opening of said first roll stand during the passage of said workpiece in accordance with said required adjustment.
 2. The gauge control system of claim 1, with said means for determining said required adjustment being operative in accordance with the mill spring modulus of each of said first and second roll stands and the workpiece plasticity in relation to each of said first and second roll stands.
 3. The gauge control system of claim 1, with said first predetermined relationship being in accordance with the following equation: GE(N) (SD(N) - LOSD(N)) + (F(N) - LOF(N))*K(N) where GE(N) is the delivery gauge error leaving said second roll stand, where (SD(N) - LOSD(N)) is the change in screwdown position of said second roll stand, where (F(N) - LOF(N)) is the change in roll force of said second roll stand, and where K(N) is the mill spring modulus of said second roll stand.
 4. The gauge control system of claim 1, with said second predetermined relationship being in accordance with the following equation: Delta SD(N-1) GE(N)*(P(N)/K(N) + 1)*(K(N-1)/P(N-1) + 1) where Delta SD(N-1) is the required adjustment of said first roll stand, where GE(N) is the delivery gauge error leaving said second roll stand, where P(N) is the workpiece plasticity in relation to said second roll stand, where K(N) is the mill spring modulus of said second roll stand, where K(N-1) is the mill spring modulus of said first roll stand, and where P(N-1) is the workpiece plasticity in relation to said first roll stand.
 5. The gauge control system of claim 1, with said second roll stand being in a condition of limited operation for controlling said gauge error such that it is desired for said first roll stand to correct said gauge error.
 6. A gauge control system for a rolling mill having at least a first roll stand followed by a second roll stand and being operative to reduce the gauge of a workpiece passed through said first and second roll stands, sAid system comprising: means for determining the second stand delivery gauge error of said workpiece leaving said second roll stand in relation to the roll force of that second roll stand and with said second roll stand being in a condition of roll opening position adjustment limit, means operative in accordance with a predetermined relationship including said second stand delivery gauge error for determining the required adjustment of said first roll stand to correct said second stand delivery gauge error by the passage of said workpiece through said first roll stand, and means for providing said required adjustment of said first roll stand during the passage of said workpiece through said first roll stand.
 7. The gauge control system of claim 6, with said determination of said required adjustment Delta SD(N-1) being in accordance with the following relationship: Delta SD(N-1) GE(N)*(P(N)/K(N) + 1)*(K(N-1)/P(N-1) + 1) where GE(N) is the second stand delivery gauge error, where P(N) is the workpiece plasticity in relation to the second roll stand, where K(N) is the mill spring modulus of the second roll stand, where K(N-1) is the mill spring modulus of the first roll stand, and where P(N-1) is the workpiece plasticity in relation to the first roll stand.
 8. A method of controlling the gauge of a workpiece passed through a rolling mill having at least a previous roll stand followed by a later roll stand for reducing said gauge, the steps of said method comprising: determining the delivery gauge error of said workpiece leaving said later roll stand during the passage of said workpiece through that later roll stand, determining the required correction of the operation of said previous roll stand for correcting said delivery gauge error during the passage of said workpiece through said previous roll stand in accordance with a predetermined relationship including the mill spring modulus and the workpiece plasticity in relation to said later roll stand, and adjusting the operation of said previous roll stand in accordance with said required correction.
 9. The method of claim 8, with said predetermined relationship being as follows: Delta SD(N-1) GE(N)*(P(N)/K(N) + 1)*(K(N-1)/P(N-1) + 1) where Delta SD(N-1) is the required correction of the operation of the previous roll stand, where P(N) is the workpiece plasticity in relation to said later roll stand, where K(N) is the mill spring modulus of said later roll stand, where K(N-1) is the mill spring modulus of said previous roll stand, and where P(N-1) is the workpiece plasticity in relation to said previous roll stand. 